1. Field of the Invention
The present invention relates generally to a semiconductor process, and more specifically, to a semiconductor process in which only one photomask is used to form layers of different depths, such as metal layers, in two recesses.
2. Description of the Prior Art
In a complementary metal-oxide semiconductor (CMOS) device, a dual work function metal gate is used for both an NMOS device and a PMOS device. Integration and process controls for the dual work function metal gate are more complicated, while thickness and composition controls for materials used when fabricating the dual work function metal gate are more precise. Fabrication of the conventional dual work function metal gate can be categorized into gate first processes and gate last processes. In a conventional gate first process for fabricating a dual work function metal gate, both an annealing process for forming a source/drain ultra-shallow junction and a silicide process are performed after forming the metal gate. After performing the annealing process with a strict heat budget, a flat band voltage (Vfb) does not increase or decrease linearly with decreasing EOT of a high-K gate dielectric layer; instead, roll-off is observed. The gate last process is developed to improve the Vfb roll-off issue, prevent leakage current generation due to re-crystallization of the high-K gate dielectric layer during high-temperature processes, and widen material choices for the high-K gate dielectric layer and the metal gate in the gate first process.
In the conventional gate last process, a sacrificed gate or a replacement gate is provided, followed by performing processes used to construct a normal MOS transistor. Then, the sacrificed/replacement gate is removed to form a gate recess. Metals are filled into the gate recess depending upon electrical needs. For example, a work function metal layer, a barrier layer and a main electrode layer are sequentially filled and formed in the gate recess.
In a current CMOS transistor process, if a work function metal layer of a PMOS transistor is formed and then a work function metal layer of an NMOS transistor is formed, a work function metal layer of a PMOS transistor cover the recesses of the PMOS transistor and the NMOS transistor. Then, two photomasks are used to remove the work function metal layer in the recess of the NMOS transistor. However, using two photomasks is complicated and costly.